In etching treatment at a mass-fabrication line for various substrates, etching is continuously conducted while filtering and recirculating an etching solution to remove foreign matter such as dust in the etching solution such that the etching solution in an etch bath is maintained clean. This applies equally to the etching of silicon nitride films with an etching solution composed of a heated phosphoric acid solution, and etching is continuously conducted by filtering and recirculating the etching solution such that a silicon compound precipitated in the etching solution is filtered off along with other foreign matter to make the etching solution clean (see, for example, JP 3-20895 B).
By the above-described process, however, the silicon compound in the phosphoric acid solution cannot be captured practically even when filtered (for example, through a filter the pore size of which is 0.1 μm) subsequent to cooling the phosphoric acid solution to room temperature, to say nothing of filtering at a high temperature. In an etching operation of silicon nitride films, an etching solution, therefore, has to be replaced periodically by a fresh supply because precipitation or deposition of the silicon compound takes place, for example, in an etch bath and a circulation route of the etching solution and depending on the concentration of the silicon compound in the etching solution, the etch selectivity of silicon-nitride-containing films, such as silicon nitride film/silicon oxide film, varies. With these circumstances in view, the present assignee have already proposed, as processes for regenerating an etching solution which contains a silicon compound, a process featuring treatment of an etching solution with hydrogen fluoride (HF) (JP 3,072,876, pages 2-3, FIG. 1) and a regeneration treatment system for an etching solution, which is suitable for use in applying the process to industrial processes (JP 11-293479 A, pages 2-5, FIG. 1).
According to the above-described regeneration process and system for an etching solution, a silicon compound in the etching solution is reacted with added HF, and a silicon fluoride as a reaction product is removed together with water vapor. Because HF employed is also a substance that affects the etch selectivity of the silicon nitride films like the silicon compound, it is essential to measure with high accuracy the concentration of HF remaining in the etching solution and to confirm and control the finish point of regeneration treatment of the etching solution (for example, the HF concentration that does not affect the etching). This end-point control is performed, for example, by sampling a portion of the etching solution. However, the end-point control is cumbersome in proceeding with continuous etching, and is a cause of cost increase.